You can get this information through the Data.Data module. There are two typeclasses which can be (have to be, I believe) derived for your data type, Data and Typeable. The Data class lets you access a representation of your type including type constructors.

To set this up, you need to import Data.Data and enable the DeriveDataTypeable extension:

    {-# LANGUAGE DeriveDataTypeable #-}
    import Data.Data

    data Codes = A0100A | A0500A deriving (Show, Data, Typeable)

Now you can get a representation of the type:
   
    λ> dataTypeOf (undefined :: Codes)
    DataType {tycon = "Main.Codes", datarep = AlgRep [A0100A,A0500A]}

Since your type is an algebraic data type (AlgRep), the DataRep contains its constructors, which you can inspect with functions from the Data.Data module.

On Mon, May 18, 2015 at 4:20 PM, Cody Goodman <codygman.consulting@gmail.com> wrote:

Can I use either template Haskell, GHC Generics, or something else to
get the list of Constructors in Codes? Then I could do `map show
codeSumTypeMembers` and get `["A0100A","A0500A"]`?

Here's an example of solving this problem manually:

data Codes = A0100A | A0500A deriving Show

codeExists "A0100A" = True
codeExists "A0500A" = True
codeExists _ = False


main = print $ codeExists xmlElementName
       where xmlElementName = "A0500A"

-- λ> main
-- True
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